Autumn is a time of transitions; hedgehogs begin hibernation, rabbits grow thicker fur, and a new group of humans leave home to start life at university. Perhaps most noticeable of all though is the changing colour of the leaves. The phenomenon of ‘leaf yellowing’ is a stunning spectacle that most of us have witnessed since childhood, but few of us know much about.
The Iroquois story of how leaves change colour is a satisfying (if gruesome) tale. They believed in a Great Bear who was killed by hunters in the heavens long ago. The blood from this event was thought to drip over the Earth’s forests, colouring them red every autumn. Luckily for the bear, this is likely not the case.
So, how do leaves change their colour? To answer this question, it is useful to understand how a leaf obtains its characteristic greenness in the first place.
A leaf is an instrumental organ for performing photosynthesis, the biological process that uses energy from sunlight to convert carbon found in the air to a form such as glucose. This requires a powerful light-harnessing pigment called chlorophyll, which makes the leaf green.
Additional pigments such as carotenoids (for orange colours – note the connection to carrots) and xanthophylls (for yellow colours) are also produced in varying ratios, making a unique leaf pigmentation pattern comparable to a human fingerprint. Having so many of these different pigments in each leaf dramatically increases the amount of light absorption able to occur.
In spring and summer, carotenoid and xanthophyll pigments are often masked by the large quantities of green chlorophyll produced by the leaves. However, as the dimmer and colder months arrive, critical biological changes occur.
The circulation of nutrients, water and minerals is cut off, ceasing the production of chlorophyll. Any that remains is quickly broken down. While chlorophyll is a great asset in warm and sunny months, it becomes an expensive liability in the autumn and winter. It becomes too cold and dark for photosynthetic chemical reactions to occur, and soon the freezing conditions will cause the leaf cells to rupture and die.
The products from the breakdown of chlorophyll are stored in the stems and roots for future use, but they also trigger the formation of red and purple anthocyanin pigments. These are characteristically found in berries and red apples to give them their red hue.
At last, with no chlorophyll to obscure them, a variety of earthy tones can bleed through the leaves, producing the beautiful landscape of yellows, oranges, reds and purples synonymous with autumn.
Hence, leaves do not change, but rather reveal their true colours in autumn.
There is a relatively simple home experiment you can do to reveal the underlying colours in any given leaf. Begin by agitating pieces of a green leaf with some rubbing alcohol in a small jar. Then cover it with cling film and stand the jar in hot water. After an hour or so, place one end of a strip of coffee filter into the jar and allow the alcohol to diffuse through the strip. It should carry with it the different pigments in the leaf, separating them out to give a beautiful pattern.
Unlike carotenoids and xanthophylls, whose colours are merely revealed in autumn, anthocyanins are actively made at this time of year which suggests they may have a more critical role than the other pigments.
In fact, anthocyanins are known to ‘shield’ leaves from the harmful effects of sunlight during the low temperatures of autumn and winter. The production of these pigments is highly dependent on the intensity of light shining on the leaves. Brighter autumn days are well-known to induce higher levels of red and purple pigments, and thus a more impressive leaf transformation.
The bright red leaves also warn insect pests that the foliage is heavily armed with chemical defences, resulting in the tree carrying fewer parasites. Unfortunately, producing this pigment is quite energy-intensive, so often trees that are more susceptible to pest invasion have redder autumn leaves but smaller fruits.
It is worth noting that, despite popular belief, areas of brown on leaves are not caused by pigmentation, but by the hardening of leaf cell walls in response to the colder weather. These processes add an extra level of colour complexity to each leaf, making no two the same.
As autumn approaches alongside the new academic year, there is much change to come. These transitions are an opportunity to show new colours, for leaves and humans alike.